1. Field of the Invention
The present invention relates to a lead frame for a semiconductor device and in particular to a lead frame for a semiconductor device which is subjected to mold packaging.
2. Description of the Related Art
Lead frames for semiconductor devices having a COL (chip on lead) or LOC (lead on chip) structure, which can realize high density assembly, are known in the art. One of them is a lead frame used in a system where a high heat-resistant insulating film coated with a thermoplastic or thermosetting adhesive layer is applied to one side or both sides of the lead frame and a semiconductor chip is mounted on the high heat-resistant insulating film under heating or under pressure.
For the lead frame in this system, it is common practice to use a polyimide film as the high heat-resistant insulating film, and, in the application of the film to the lead frame, die stamping application is used. Specifically, a film in a reel form is stamped by means of a die into a desired shape and then applied to the lead frame under heating or under pressure.
According to this method, the film is stamped by means of a die, and the stamped film is then applied to the lead frame in its desired position. Therefore, the amount of the tape used is large, resulting in increased cost. Further, since stamping of the film produces a waste film, the material is wasted. Furthermore, absorption of moisture into the polyimide film leads to a fear of package cracks being created.
In order to eliminate such inconvenience, a method has been proposed which comprises: coating an adhesive onto a lead frame in its area where a semiconductor device is to be mounted; and jointing (bonding) the semiconductor device to the lead frame with the aid of this adhesive.
For the coating of the adhesive, a device comprising a combination of an X-Y robot with a dispenser is generally used to coat the adhesive onto a lead frame in its area where a semiconductor device is to be mounted (for example, a front end of an inner lead). in particular, when an adhesive is applied to the front end of the lead, a spot coating method is used. In this method, what is required is only to coat a required amount of the adhesive onto the lead frame. Therefore, no waste material is produced, and no expensive die is required, advantageously resulting in reduced production cost.
A varnish adhesive (for example, a solution of an adhesive resin in a solvent) is coated by ejecting the adhesive through a capillary needle (or nozzle) by means of air pressure onto a lead frame in its predetermined area while moving the needle (or nozzle) on the lead frame.
A lead frame using the conventional adhesive coating means poses problems including (i) that it is difficult to provide a thick coating and (ii) that a variation in coating thickness occurs when leads to be coated with an adhesive do not have an identical or substantially identical shape and lead spacing. A large variation in coating thickness leads to a fear that, in bonding a semiconductor chip to a lead frame, thickly coated leads alone are bonded to the semiconductor chip with thinly coated lead remaining unbonded to the semiconductor chip. The lead which could not be bonded to the semiconductor chip becomes floated and, hence, instable, adversely affecting the subsequent wire bonding.
Even though leads have an identical or substantially identical shape and lead spacing, a lead spacing smaller than the lead width makes it difficult to form a thick coating although an even coating could be formed. In this case, conditions for bonding a semiconductor chip to the lead frame cannot be easily determined, and, in addition, large impact occurs at the time of bonding. Further, when the distance between the semiconductor chip and the lead frame is excessively small, a resin cannot be flowed into between the leads, leading to a fear of a cavity being created and, in addition, adversely affecting the reliability of the semiconductor device.